1. Field of the Invention
The present invention relates to an organic electroluminescent device and a display apparatus including a plurality thereof.
2. Related Background Art
Recent years have seen progress in the development of organic electroluminescent devices (hereinafter, referred to as “organic EL device”).
Organic EL devices have at least one organic layer between electrodes. The organic layer includes at least a light-emitting layer, wherein the organic layer structure can be, for example, a three-layer structure consisting of a hole-transporting layer, a light-emitting layer and an electron-transporting layer, or a two-layer structure consisting of a hole-transporting layer and a light-emitting layer.
Japanese Patent Application Laid-Open No. 2002-050483 (corresponding to U.S. Patent Application Publication No. 2002/0146589) discloses a light-emitting layer which has three kinds of organic compound. The paragraphs [0038] and [0039] referring to FIG. 3 of Japanese Patent Application Laid-Open No. 2002-050483 (corresponding to the paragraphs [0048] and [0049] referring to FIG. 3 of U.S. Patent Application Publication No. 2002/0146589) describes the following.
An energy transfer takes place from the lowest excitation singlet state of the third organic compound to the lowest excitation singlet state of the first organic compound, and transits to the lowest triplet state by intersystem crossing. Further, an energy transfer takes place from the lowest excitation triplet state of the third organic compound to the lowest excitation triplet state of the first organic compound. Thereafter, an energy transfer takes place from the lowest excitation triplet state of the first organic compound to the second lowest excitation triplet state of the second organic compound, and after the transition to the lowest excitation singlet state by reverse intersystem crossing, fluorescence is emitted in the process of returning to the ground state.
The quantitative relationship among the first, second and third organic compounds of Japanese Patent Application Laid-Open No. 2002-050483 is unclear. More specifically, Examples in this application disclose that a first organic compound is a phosphorescent material which does not emit light and a second compound is a fluorescent material which does emit light. Further, it is disclosed in the Examples that the proportion of these two compounds is respectively 0.02 mg and 0.10 mg, whereby it is understood that there is more of the fluorescent material which emits light. However, Japanese Patent Application Laid-Open No. 2002-050483, does not clearly specifies as to what the third compound is, and whether the third compound is quantitatively greater than the first and second compounds. Therefore, there is no basis for supposing that the light-emitting layer containing these first, second and third compounds is a so-called host-guest type light-emitting layer.
In addition, the organic EL device described in Japanese Patent Application Laid-Open No. 2002-050483 requires further improvements in terms of emission efficiency, as well as lifetime and power consumption as a device.
In view of the above, it is an object of the present invention to provide an organic EL device having greater efficiency and a longer lifetime.